Laser liquid level gauge with diffuser

Abstract

The liquid level sensor includes a modulated laser diode and collimating optics for collimating the diode output to produce an optical measurement signal beam transmitted to the fluid boundary through a diffuser to diverge the beam. A detector detects reflections of the beam from the fluid boundary to produce an output signal which is amplified and inverted to drive the modulator. The frequency of the modulation, which depends on the distance to the fluid boundary, is measured and the distance is derived from that measurement.

I claim: 
     
       1. In an apparatus for measuring the level of a light-reflective fluid boundary in a vessel, including a transmitter for transmitting an optical measurement signal toward the fluid boundary, a receiver for detecting a reflection of the signal from the fluid boundary, and control circuitry for modulating the transmitted measurement signal and processing the received reflections for determining distance between the transmitter and the fluid boundary as a function of a parameter of the modulation, the improvement comprising: a diffuser disposed between the transmitter and the fluid boundary for diverging the optical measurement signal. 
     
     
       2. The apparatus of claim 1, wherein the control circuitry includes means for determining the distance between the transmitter and the fluid boundary as a function of the modulation frequency. 
     
     
       3. The apparatus of claim 1, wherein the transmitter transmits a collimated optical signal. 
     
     
       4. The apparatus of claim 3, wherein the transmitter includes a laser light source. 
     
     
       5. The apparatus of claim 4, wherein the transmitter includes a laser diode for generating an uncollimated optical signal, and collimating optics for collimating the optical signal for the laser diode, the control circuitry determining the distance between the laser diode and the fluid boundary. 
     
     
       6. The apparatus of claim 3, wherein the diffuser diffuses the collimated optical signal into a conical signal having a half angle of about 1°. 
     
     
       7. The apparatus of claim 1, wherein the control circuitry includes means cooperating with the transmitter to generate a continuous optical signal measurement beam of varying amplitude. 
     
     
       8. The apparatus of claim 1, wherein the vessel has a windowed aperture through which the optical measurement signal and the reflections pass. 
     
     
       9. Apparatus for measuring the level of a light-reflective fluid boundary in a vessel, said apparatus comprising: a transmitter for transmitting an optical measurement signal beam toward the fluid boundary; a diffuser disposed between the transmitter and the fluid boundary for diverging the optical measurement signal beam; a detector for generating an output signal in response to detection of a reflection of the signal beam from the fluid boundary, the detector output signal being related to an amplitude of the detected reflection; a modulator coupled to the transmitter and to the detector for causing the transmitter to continually transmit the measurement signal beam at a first amplitude while the amplitude of the detected reflection is at a second amplitude, and causing the transmitter to continually transmit the measurement signal beam at a second amplitude while the amplitude of the detected reflection is at the first amplitude; and means for measuring the frequency of modulation of the signal beam, the frequency being indicative of the distance from the transmitter to the fluid boundary. 
     
     
       10. The apparatus of claim 9, wherein the transmitter transmits a collimated optical signal beam. 
     
     
       11. The apparatus of claim 10, wherein the transmitter includes a laser light source. 
     
     
       12. The apparatus of claim 11, wherein the transmitter includes a laser diode for generating an uncollimated optical signal, and collimating optics for collimating the optical signal for the laser diode, the control circuitry determining the distance between the laser diode and the fluid boundary. 
     
     
       13. The apparatus of claim 12, wherein the diffuser diffuses the collimated optical signal into a conical signal having a half angle of about 1°. 
     
     
       14. The apparatus of claim 9, wherein the vessel has a windowed aperture through which the optical measurement signal and the reflections pass. 
     
     
       15. A method for measuring the level of a light-reflective fluid boundary in a vessel comprising the steps of: transmitting an optical measurement signal beam from a source toward the fluid boundary; diffusing the optical measurement signal beam before it reaches the fluid boundary; detecting a reflection of the signal beam from the fluid boundary; modulating the signal beam in response to the detected reflection, the modulating step including the steps of: (a) causing the source to continually transmit the measurement signal beam at a first amplitude while the amplitude of the detected reflection is at a second amplitude, and   (b) causing the source to continually transmit the measurement signal beam at the second amplitude while the amplitude of the detected reflection is at the first amplitude; and measuring a frequency of modulation of the signal beam, the frequency being indicative of a distance from the source to the fluid boundary.     
     
     
       16. The method of claim 15, wherein the transmitting step includes transmitting a collimated optical signal beam. 
     
     
       17. The method of claim 16, wherein the transmitting step includes generating an uncollimated beam at the source and collimating the uncollimated beam. 
     
     
       18. The method of claim 16, wherein the diffusing step includes diverging the collimated beam to a conical beam having a half angle of about 1°.